The present invention relates in general to communication systems and, in particular, to a multi-frequency radio data communication system wherein channel selection is effected at all times with a view towards maintaining a given threshold level of service quality and balanced loading.
A typical radio data communication system may comprise a centrally located Network Control Processor (NCP) connected by wire line, usually a leased line, to a plurality of remote RF transmitting stations defining the coverage for a given area. The NCP is also connected to, and is controlled by, a host computer. The individual RF stations may include a local General Communication Controller (GCC) and an associated transmitter/receiver. The individual RF stations are all interconnected to the central NCP to thus form the particular data network which communicates with a plurality of portable/mobile (remote) data terminals transportable throughout the coverage area.
As may be appreciated, there are certain significant differences in such radio data systems as compared to a more well-known conventional two-way radio voice communication system. Normally, such radio data systems require two separate channels, an inbound channel on which the portable/mobile terminals communicate with the central NCP, and an outbound channel on which the NCP communicates selectively with the individual portable/mobile terminals. Another distinguishing aspect is that the individual portable/mobile terminals must, when they are to make a transmission to the central NCP, contend with the other remote radio terminals in the system for access to the inbound channel.
The system portable/mobile terminals monitor the outbound channel in a manner determined by an operating protocol. In one known arrangement, the portable/mobile terminals monitor the outbound channel for an indication of any activity by others and, if sensed, do not transmit at that time, but retry later, usually on a random basis. To aid the portable/mobile terminals in determining the presence of channel activity on the inbound channel, the NCP includes a stream of bits, termed "busy bits," embedded in the messages constituting the outbound channel stream, whenever the NCP is in fact receiving a transmission on the inbound channel. This protocol regulates radio traffic and is conventionally termed a CSMA, or "Carrier Sense, Multiple Access," protocol. There are variations of this CSMA operating procedure to accommodate various operational needs of the radio data systems but need not be described in any further detail regarding the merits of the present invention.
The foregoing arrangement works entirely satisfactory for radio data communication systems operating on a single frequency. In multi-frequency systems, however, other factors come into play. In such multi-frequency systems, for example, it is for the portable/mobile terminal to determine, and insure effective operation on a particular channel, among a choice of channels, which will effect acceptable service. Accordingly, the terminal must be capable of monitoring the various channels in terms of signal/service quality and change channels if need be (when service quality falls below some set threshold level).
There are a number of variables that make this monitoring task somewhat complex and difficult. For example, the terminals can move in and out of the defined coverage area and therefore the service quality available to the terminal on any particular channel will probably change over time. Further, the received RF signal may well suffer from the well-understood multipath fading effects, so that a reasonable statistical sample may well be required in order to make an effective guess at signal level. Moreover, there may be more than one RF station serving a particular geographical area on the same channel. Received signals on some of these channels might well appear to be weak while others strong. Further, if the system includes multiple stations on the same channel which may be keyed by the central NCP controller, a condition may occur in which RF collisions are noted. Finally, it must be kept in mind that in selecting appropriate channels, it is desired that minimal channel capacity be dedicated to this function since the main purpose of the radio data communication system is to transmit users' data messages.
In any event, to monitor the channel for service quality, the portable/mobile terminals must have information to monitor, which necessarily must be provided by the centrally located NCP on the outbound channel. One known method employed in the past has been to utilize a Signal Quality Message (SQM) as part of the outbound channel stream. This message was devised to contain a predetermined bit pattern so that when received by the various remote terminals, the bit errors could be counted directly. However, such SQM's occupy a fair amount of the outbound capacity--on the order of about 18% (per station). It will well be appreciated that this technique does not appear conducive to multi-frequency systems with multiple co-channel stations. If there were only five such stations on a channel, as an example, the SQM technique would require five times 18%, or about 90%, or just about the entire outbound capacity of the channel. Obviously, this is less than desirable.
Accordingly, what is needed is an arrangement for use in a multi-frequency radio data communication system wherein the individual portable-mobile terminals may nevertheless continuously and effectively monitor and, in fact, determine the service quality of each of the available channels without unduly affecting (wasting) system capacity. Moreover, such terminals when faced with service quality falling below a set threshold level, should be capable of selecting a new channel to overcome such deficiency. Short term information should be obtainable to combine with longer term past data to obtain a good statistical estimate for the quality of the channel so selected.